Transducer of mechanical vibrations into electrical oscillations



. -May 19, 1959 Filed Jan. 10, 1957 C. BQUBET ET AL TRANSDUCER OF MECHANICAL VIBRATIONS INTO ELECTRICAL O SC ILLATIONS 2 Shge ts-Sheet 1 MOE/WEES avfiqbaas IBaUBE 714, fl/vpeg, 4 7, JDQPU/S May 19, 1959 c. BOUBET ET AL 2,837,660

TRANSDUCER OF MECHANICAL VIBRATIONS INTO ELECTRICAL OSCILLATIONS Filed Jan. 10, 1957 2 Sheets-Sheet 2 Fig.2

United States PatetitO TRANSDUCER OF MECHANICAL VIBRATIONS INTO ELECTRICAL OSCILLATIONS Charles Bouhet, Paris, and A ndr Louis Marie Du puis," Racquinghem, France The present invention relates to the realization of a transducer of mechanical vibrations into electrical oscillations by varying the impedance of an oscillating circuit which modifies the feedback rate of a high-frequency oscillator.

This transducer is mainly characterized by the fact that it is constituted by a modulator-oscillating circuit com prising an induction coil shunted by a condenser having two sets of plates, one fixed and one vibrating, by a triode tube having a high transconductance, whose cathode is connected to the aforesaid coil by an appropriate tap on the coil, the oscillating circuit of the triode tubecomprising a coil in series with the plate of the tube and tuned by a second condenser to a frequency slightly higher than the natural resonant frequency of the unexcited modulator circuit, by a coupling coil inserted between the grid of the tube and ground, by a resistance shunted by a third condenser and intended to cathode bias the tube, by a coil inserted between the tap of the modulator circuit coil and the gridof the tube, by a second resistance in series with the tube oscillating circuit coil, and by a fourth and a fifth condenser one acting as a by-pass to ground and the other as a coupling to the audio-frequency circuit.

The triode bias is chosen so as to be near the cut-ofi portion of the dynamic characteristic of the tube.

The coil inserted between the tap of the modulator circuit coil and the tube grid is intended to compensate the drift of the oscillating frequency.

Over-compensation by increasing the value of the aforesaid coil may be desirable as it inverses the direction of frequency variation. Thus, the sensitivity is increased and a compensation is obtained for the distortion brought by the non-linearity of the impedance variation of the modulator circuit as a function of the distance separating the platesof the condenser of this circuit.

The low-frequency component of the high-frequency oscillation resulting from the variations of the triode average current appears across the terminals of the second resistance.

The fourth condenser and the second resistance constitute a low-pass filter which eliminates the high-frequency component.

The fifth condenser transmits to the normal amplifying circuits the low-frequency voltage appearing across the terminals of the second resistance.

An embodiment of the invention is shown schematically and by the way of an example on the appended drawings in which:

Fig. 1 represents the transducer schematic diagram.

Fig. 2 represents a practical embodiment of this transducer.

According to Fig. l, the modulator circuit is constituted by an induction coil 1 shunted by a condenser 2 comprising one fixed plate arm and one vibrating plate arm. The coil 1 is connected by a suitable tap 3 to the cathode 4 of a high transconductance triode tube 5. The

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. oscillating circuit of the tube 5 is constituted by a coil 6 in series with the tube plate 7; this coil 6 is tuned by a condenser 8 to a frequency slightly higher than the natural frequency of the uuexcited modulator circuit. A coil 9 is inserted between grid 10 and ground 11 without any need of the usual by-passing condenser.

. The tube 5 is biased in its cathode 4, by a resistance 12 shunted by a condenser 13;

Between the tap 3 of the modulator circuit and the grid 10, a coil'1'4 isjinserted. A resistance" 15 is placed in series with the coil 6. Across the terminals of this resistance appears the low-frequency component of the highfrequency oscillation resulting from the variations of the average current of the tube 5. A condenser 16 acts as a by-pass to ground while another condenser 17 acts as a coupling between the output and the audio-frequency circuits BF.

Due to the modulator circuit 1, 2, feedback is applied to the tube 5 and the rate varies according to the resonant frequency, therefore according to the displacement amplitude of the mobile plate of the vibrating condenser 2 with respect to the fixed plate. The feedback voltage proportional at each moment to the high frequency current intensity, modifies the value of the effective transconductance of the tube 5 and consequently the amplitude of the oscillations produced by this tube.

The non-excitation bias of the tube 5, is adjusted so as to be located near a bend of the tube dynamic characteristic. The HP oscillation amplitude variation results in a corresponding variation of the plate current average intensity, which restores the modulation audio-frequency component across the resistance 15 in series with the plate circuit. This resistance with its shunting condenser 16 acts as a low-pass filter eliminating the HF component, whose further amplification is useless.

To the main phenomenon explained above is added a secondary phenomenon of frequency drift which must be corrected, if the proposed system sensitivity must be kept at its maximum. On the other hand, the capacity variations of the vibrating condenser 2 are not a linear function of the angular displacement of the movable plate with respect to the fixed plate, also an oscillatory circuit impedance is not a linear function of its tuning capacity; consequently an amplitude distortion arises which must be compensated by an inverse distortion of the modulator-detector.

By way of example, experiments show that, when the resonant frequency of the modulator circuit 1-2 varies from 16.4 to 1 6.8 megacycles/sec., the oscillation frequency varies from 17 to 17.2 megacycles/sec. The feedback rate is thereby decreased, as well as the value of the usable audio-frequency voltage.

It is possible to compensate this spurious frequency drift by a correction coil 14 connected between the grid 10 and the cathode tap 3. For a certain value of the correction coil 14, the frequency drift disappears. If the value of the correction coil 14 is increased, the frequency drift reappears, but in the opposite direction. When the resonant frequency of the modulator circuit 1--2 varies from 16.4 to 16.8 megacycles/sec., the oscillation frequency then varies from 17 to 16.9 megacycles/sec. This phenomenon is favorable, since it increases the feedback rate as Well as the usable audiofrequency voltage.

Furthermore through a judicious setting of this overcompensation, a curve of the dynamic characteristic is obtained, which corrects exactly the non-linear distortions resulting from the use of the vibrating condenser.

Thus, a high sensitivity transducer is made available which subjects the material whose vibration must be studied to a minimum of mechanical stresses and which p is characterized with respect to the already existing systems by the following advantages:

(1) Lower background noise,

(2) High stability,

(3) Perfect compensation for the non-linear capacity variation of the vibrating condenser,

' (4) Great facility in the adjustments.

This device can be applied to any kind of transducer of mechanical vibrations into electrical oscillations, phono-pick-ups, microphones, vibration pick-ups Similarly the impedance variation of the modulator-oscillating circuit can be obtained by other means such as a variation of the inductance, either directly or indirectly by absorption. Similarly the oscillatory circuit may use any method which will permit the obtention of sustained oscillations (transistors for example). ,The device is not limited to the demodulation of the HF oscillations by the mentioned method; it is possible to receive and to detect the modulated HF oscillations at some distance from the transmitting point (for example an autonomous microphone, Without any connection with the amplifying and transmitting circuits).

Fig. 2 represents a practical embodiment of the transducer intended to reduce to a minimum the oscillator noise and to increase the output voltage while reducing the background noise. It applies to a tuned-grid oscillator using a double triode tube. connected in cascode.

This tube comprises eifectively two triodes 18 and 19; the triode 18 has a plate 20, a cathode 21 and a grid 22, while the triode 19 has a plate 23, a cathode 24 and a grid 25: these two triodes are connected together to form a cascode coupling.

The modulator circuit is constituted by a coil 26 shunted by a condenser 27 and connected by an appropriate tap 28 to the cathode 21. The capacity of the condenser 27 varies according to the distance between its two plates, one fixed, the other one subjected to the mechanical vibrations to be transformed into variations of the electric voltage. The assembly 26, 27 is an antiresonant circuit whose impedance varies according to the value of the capacity of the condenser 27. These impedance variations give rise to a current feedback in the cathode 21 of the triode 18 connected to the tap 28 of the coil 26, thereby realizing a low-impedance connection through a shielded cable 29.

The grid oscillatory circuit which determines the unmodulated frequency of the high frequency oscillations is constituted by the coil 30 shunted by a condenser 31 and connected on one hand to the grid 22 and on the other hand to ground through a condenser 32 shunted by a resistance 33.

A coupling coil 34 is connected in series on one hand to the plate 23 and on the other hand to a resistance 35 being connected to voltage (for instance +200 volts).

A condenser 36 and a condenser 37 are connected in parallel on the circuit coil 34-resistance 35, the condenser 36 being connected to ground and the condenser 37 to a low-frequency amplifier circuit BF.

A resistance 38 by-passed by a condenser 39 is connected between the coil 26 and the cathode 21 to determine the bias of the triode 18. A resistance 40 by-passed by a condenser 41 is connected in series between the grid 25 and the ground and a resistance 42 is connected in series between the grid 25 and the voltage HF. The resistance bridge 40-42 connected between the voltage HT and the ground determines the bias of the triode 19.

The value chosen for the various bias locates the triode assembly 1819 in the neighbourhood of a bend of its dynamic characteristics.

An adjustable condenser 43 is connected in series between the plate 20 and the condenser 32. The plate 20 is connected to the cathode 24 of the triode 19.

The natural unmodulated resonant frequencies of the modulator circuit 2627 and grid circuit 3031 as well as the mechanism of amplitude modulation through a variation of the oscillatory tube effective transconductance have been made clear from the preceding.

In the plate circuit of tube 19, the BF component of the modulated HF voltage appears across the terminals of the resistance 35. This voltage is fed through condenser 37 to the input of a power amplifier the condenser 36 together with the resistance 35 constitutes a low-pass filter eliminating the high-frequency component which it is useless to amplify further.

In this coupling, no correction coil is used. The cancellation, or even the direction reversal of the frequency drift mentioned above is obtained by a neutralizing condenser, owing to the condensers 32 and 43 which carry to the lower part of the grid coil a fraction of the alternating voltage of the plate 20 of the tube 18, itself connected to the cathode 24 of the tube 19.

What we claim is:

A translation system comprising a modulator circuit including an induction coil and a condenser, said coil being shunted by said condenser, said coil having a tap, said condenser having a fixed plate and a vibrating plate; a triode having a cathode, a grid, and a plate, said cathode being connected to said tap; an oscillating circuit including a coil, said coil being connected in series with said plate, and a condenser means for tuning said coil to a frequency slightly higher than the natural frequency of the modulator circuit; a coupling coil connected to the grid and to ground; a resistance connected to said tap and to said cathode, said resistance being shunted by a condenser; a coil connected to said tap and to said grid; a resistance connected in series with said oscillating circuit coil, and a shunting condenser connecting said latter coil with ground.

References Cited in the file of this patent UNITED STATES PATENTS 2,416,215 Rath Feb. 18, 1947 2,436,129 Weathers Feb. 17, 1943 2,682,579 Weathers June 29, 1954 

